1. Field of the Invention
The present invention relates to a metal halide lamp and an automotive headlamp apparatus incorporating the same.
2. Description of the Related Art
The present inventors have developed a metal halide lamp suitable for an automotive headlamp that substantially contains no mercury (conveniently referred to as a mercury-free lamp, hereinafter). The metal halide lamp has electrodes each having a shaft part 0.3 mm or more in diameter and a planar tip, the distance between the electrodes being 5 mm or less. And, the metal halide lamp has, sealed in a hermetic vessel as a discharge medium, halides of metals including a light-emitting metal in an amount of 0.005 mg/mm3 of the inner volume of a hermetic vessel or more and a xenon gas at 5 atmospheres or more at 25° C. The above-described halides additionally include a halide of a metal that has a high vapor pressure and emits no or a relatively little visible light, the halide being intended to provide a lamp voltage in place of a mercury vapor. The mercury-free lamp is turned on with a lamp power of 60 W or lower.
For example, in Japanese Patent Laid-Open No. 5-57696, there is described an electrode of a high intensity discharge lamp, such as a metal halide lamp, whose tip is thicker than the shaft part thereof and is spherical in shape (Related Art 1).
Furthermore, in J. Phys. D: Appl. Phys. 33 (2000) 367-374. Printed in the UK, there is described a cathode of a high intensity discharge lamp whose tip has a curved surface having a radius of one-half of a diameter of the shaft part of the cathode (Related Art 2).
In contrast to the metal halide lamp having mercury sealed therein (conveniently referred to as a mercury-containing lamp, hereinafter), since the mercury-free lamp described above has no mercury sealed therein, it cannot provide a thick arc, which would be provided in the presence of a mercury vapor. This means that the resulting arc is relatively narrow, and thus, unstable, so that a luminance flicker tends to occur.
A halide vapor intrinsically serves to make discharge narrow and unstable. Furthermore, in the mercury-free lamp, a halide of a metal having a high vapor pressure may be sealed to provide a lamp voltage and/or correct a chromaticity. Therefore, the mercury-free lamp has a halide vapor pressure several orders of magnitude higher than that of the mercury-containing lamp. The higher vapor pressure of the halide of the metal results in a narrower, and therefore, more unstable arc, so that a luminance flicker tends to more readily occur.
In addition, in the mercury-containing lamp, HgI2 is produced from mercury, and therefore, a free halogen is unlikely to occur. To the contrary, in the mercury-free lamp, since no HgI2 is produced, a free halogen is easy to occur. Since the free halogen highly adsorbs electrons, the resulting arc is narrow, and thus, a luminance flicker tends to occur.
As for the conventional electrode configuration of the metal halide lamp, according to the Prior Art 1, the electrode has a tip which has a curved surface and is wider than the diameter of the shaft part thereof. In addition, the lamp according to the Prior Art 1 is a mercury-containing lamp and is not intended to reduce the luminance flicker. According to the description of the Prior Art 1, the above-described electrode configuration is intended to be adapted for various current values in various operation phases of the high-pressure xenon/metal halide lamp.
The electrode configuration according to the Prior Art 2 is the same as that according to the invention described later. However, in the Prior Art 2, a temperature distribution of the electrode is only calculated, and no mention is made of the relationship between the configuration and the luminance flicker and the mercury-free lamp.